Apolipoprotein E-Containing Lipoproteins and Lipoprotein Remnants in Experimental Canine Diabetes

The effects of diabetes on plasma lipoproteins were examined in a cohort of control and streptozocin-alloxan diabetic beagles fed either standard rations or an atherogenic cholesterol-supplemented diet. Lipoprotein cholesterol, triglyceride, and retinyl ester concentrations were measured in fractions separated by density gradient ultracentrifugation. Individual lipoprotein classes and apolipoproteins were assessed by electrophoresis. Postheparin plasma lipoprotein triglyceride lipase activities were also examined. In the absence of added dietary cholesterol, diabetic animals became hypercholesterolemic with relatively increased low-density (LDL) and decreased high-density (HDL) lipoprotein cholesterol concentrations. Apolipoprotein E-containing beta- to alpha 2-migrating HDL1 (HDLc) appeared in rho = 1.020-1.080 g/ml subfractions, whereas alpha 1-migrating typical HDL (rho = 1.06-1.21 g/ml) was reduced. In comparison to nondiabetic cholesterol-fed animals, diabetic cholesterol-fed animals had increased cholesterol (but not triglyceride) concentrations in very-low- and intermediate-density classes. These classes contained retinyl esters and low-molecular-weight apolipoprotein B (components of intestinal lipoprotein remnants) as well as apolipoprotein E and high-molecular-weight apolipoprotein B. These findings could not be explained by decreased postheparin plasma lipoprotein lipolytic activities. Increased plasma concentrations of HDLc in poorly controlled diabetic dogs may reflect a pathologic disturbance in the excretory limb of cholesterol transport from peripheral cells to the liver. In addition, exaggerated retention of lipoprotein remnants in cholesterol-fed diabetic dogs may contribute to increased delivery of cholesterol to extrahepatic tissues. This model appears to be suitable for physiologic studies of the effects of diabetes on reverse cholesterol transport.